In an impact modifier, the impact resistance comes from the constituent crosslinked, low glass transition (Tg) rubber. The characteristics of this rubber directly affect the ability of the modifier to provide ductility in a base polymer during impact. One of these characteristics is crosslink density because it controls the stiffness of the rubber and its ability to cavitate. Often, in an impact modifier, the crosslink density is controlled by the addition of a crosslinker into the low Tg polymer. The distribution of the crosslink density is directly proportional to the distribution of the crosslinker in the rubber. The rubber is also often made with a shot process, whereby all the monomer is charged to the reactor and then its polymerization is initiated, in order to keep the standing monomer concentration high and the branching in the rubber low; therefore, the crosslinker distribution, and thus, the crosslink density distribution is entirely dependent on the reactivity ratios of the rubber monomer and crosslinker.
Butyl, ethyl, 2-ethylhexyl and other acrylic esters are often the rubber monomers of choice, and either a multi-functional acrylate or allyl methacrylate (ALMA) are often the crosslinkers of choice. ALMA has the advantage of different reactivities of the allyl double bond and the methacrylate double bond. Therefore, ALMA produces less backbiting loops in the rubber and more graftlinks to the shell. However the higher reactivity of the methacrylate double bond in ALMA compared to that of the acrylate double bond causes the crosslink density to be high at the beginning of each shot and then decrease to zero at the end of the shot. Thus, there is a need for a more effective crosslinker to use in an acrylic impact modifier made by the shot process.